i wish to convert a tx/rx wall charger from 70ma out on the tx line to 110ma out as the one supplied by cyberheli hasnt got the power to charge my 9z pack 1100ma even if i leave it on indefinately. in the uk futaba doesnt do higher ma outputs on wall chargers. ive opened it up and the tx line has 15volts coming off that side of the transformer one line goes to a large diode then to a large resistor. the other line goes to a small resistor and the LED seen on the outside then to the output.
How do i figure out to to increase the ma output?

You would have to change your resistance values aruond. Amperage is directly proportionate to voltage and inversly proportionate to resistance. I just did some math and found that with 15 volts, a resistance of 136 ohms will give you your .11 amps. see, look:
15V / .11A= 136.3636363636 ohms. so I think if you find out wich resistor is the current limiting resister you wil just need to change that out.

yep but isnt it more complicated than that as u assume the voltage is fixed by the transformer if i reduce the resistance would the voltage not be affected as well as the current. are tx packs supposed to be charged at higher volts and why?

i beleive the transformer output will remain constant. that is determined by the numbers of windings in the coils. i am not exactly shore if i am right on what i said, it sounds right in paper, change out the resistor and see what happens. what value is the big resister in there now?

havent a clue but il pop it out 2nite and have a look. if only i can find my multimeter.....

You should be able to tell by the coloured bars that are on it! have a look here Resistor chart . Why dont u just do what i do and use ure field charger? That way you can charge it at whatever level you require.

Is there? Opps i use my charger to charge my tx battery pack as it is 2300mah so is it fooked then? LOL

This Heli Will Self Destruct In Five Seconds!!

ive taken a picture. if anyone cares to pm me their email address they can see for themselves.
one of the tx wires the negative is attached to a to a big grey thing thats marked R4 on the board so persumably this is a resistor, then it goes to the tx. the other wire coming out of the transformer goes to a diode then goes to a resistor with a yellow band thenblack band then green band then brown band then part of this goes to the led but part filters straight out the positive tx wire.. the other leg of the led takes a lead straight from the diode before it hits the first leg of the resistor

the big grey thing on the first line appears to have only 3 bands but mayb 4 bottom band brown, next appears blank but could be a faded white?? next black then finally yellow.

Washout

You are quite right, it is more complicated than that. If the transformer can only supply 70 mA then as you take more current from it, it will drop its output voltage and get hot.

The other complication is that the voltage being limited by the resistor is not 15v but 15v minus the battery voltage, nominally 9.6v so theoretically a 49 ohm resistor would be correct, practically a 47 ohm is ok. A 1 watt resistor will be big enough but you may want to go bigger to be sure it wont get too hot.

If you do this, keep an eye on the transformer temperature. When the battery is flat the current will be high and the transformer will get hot. As the battery gets charged, the terminal voltage will rise and the current will drop, reducing the load on the transformer.

Hi Peter haha so bees do what now (a quote from Homer)
im a bit dumb i understand what ur saying but why are their 2 resistors is the smaller one then 4K ohms so "big" to stop the led being fried. fair enough then the grey thing whats that 1.9Mohms doesnt make sense im going to desolder both and check them and then do the sums. But i take ur point. its interesting....
Any electronics gurus out there.....

Hi, ud think that i did 2 but ive had it on for 36hours and the voltage never goes past 11.1 it should read 11.6 immeditely when charged immediately checking it.
the polarity definately does correspond as its a futaba charger supplied with the 9z. and it does charge it but no where near what it should.
ive read somewhere if the charger isnt putting out enough current it wil never charge the pack. perhaps one of the resistors is too big but Futaba rate it at 70ma 9.6vf so??????

I agree that 11.1 sounds fine.
I usually get about 11 on the pcm 10 after a charge.
Paul.

in the macgregor manual for my jr642 which also has a 9.6v pack it says when taken off charge the display should read at least 11.2 volts. from my experiance charging the 642 it reads 11.6 and it isnt just my experiance ive read that number mentioned in several places. Try it with yours fully charge it and turn it on and see.
I think ill turn on the tx and see how long i get out of it.
my opinion is as the pack charges it oposes the transformer and it cant put the current in then or does it slowly.
i might just start fast charging it with my supernova but why wreak the pack when i can take the time to do it overnight. after all i did pay 750 pounds for it from cyber last year (with 4 9252)

U can charge it safely on a field charger just dont go mad with the charge rate 1c is a good max ie 1.6A on a 1600mah battery. 11.1 will be fine the highest i have seen mine was 11.4 which it does not stay at for very long. Why go through all this hassle when u have no real problem.

Ive turned it on(with the aerial up) well c how long it lasts. il report later.

One of the resistors is, as you srmise, to limit the current to the LED. It needs about 2.2v at 10mA or less so it has the smaller resistor.
If you have it apart, measure the value of the two resistors and let me know. No need to unsolder them.

Peter the plot thickens
ive checked the resistances soldered then i unsoldered them out of interest the small resistor on the led line is only 9.3ohms and the large one is just 15.5 ohms yes thats all. and by the way they were identical soldered and unsoldered but couldnt believe how low they are.
see if you or anyone can figure it out but its really weird.

the above may all be acedemic as its been one hour and the 9z is holding at 9.9 volts so maybe it does work?

by the way this is the first time ive really used the 9z as ive bee n using the 642 its.........lighter haha

good luck in figuring the above out and yes the multimeter is a good one.